This invention relates to lower-frequency radars (below microwave), and embodies improvements by way of compact electronics and antennas, efficient signal waveforms and their digital generation/processing, and direction-finding (DF) angle measurements.
2. Cross-Reference to Related Applications
______________________________________ Field of Search 432/107, 432/131, 432/132, 432/139, 432/195, 432/196 343/726, 342/728, 343/742 U.S. Pat. No. 3,882,506 1975 Mori et al. 343/728 4,053,884 1977 Cantrell and Lewis 432/132 4,172,255 1979 Barrick and Evans 432/107 4,309,703 1982 Blahut 432/132 4,433,336 1984 Carr 343/728 4,896,159 1990 Sabatini et al. 432/131 5,023,618 1991 Reits 432/196 ______________________________________
3. Other Publications
Barrick, D. E. (1973), FM/CW radar signals and digital processing, NOAA Tech. Report ERL 283-WPL 26, U.S. Dept. of Commerce, Boulder, Col.
Prandle, D. & D. K. Ryder (1985), Measurement of surface currents in Liverpool Bay by high frequency radar, Nature, vol. 315, pp. 128-131.
Lipa, B. J. & D. E. Bartick (1983), Least-squares methods for the extraction of surface currents from CODAR crossed-loop data: Application at ARSLOE, IEEE J. Oceanic Engr., vol. OE-8, pp. 226-253.
Lipa, B. J., & D. E. Barrick (1986), Extraction of sea state from HF radar sea echo: Mathematical theory and modeling, Radio Sci., vol 21, pp. 81-100.
4. Description of Prior Art
Lower-frequency radars operating in the MF, HF, and VHF bands, are useful for a number of applications. Among them are ocean wave and surface current monitoring, as well as detection of discrete targets, e.g., aircraft, ships, misstics, etc. The advantages are: (i) their ability to see beyond the horizon, in both skywave and surface-wave propagation modes; (ii) the comparable size of their wavelength with scattering target dimensions, allowing resonance with the target; and, (iii) lower data rates (resulting from the low frequency) permit easy digital signal generation and processing. The radars considered here operate typically three orders of magnitude lower in frequency than the much more more common microwave radars. Their consequent disadvantages compared to microwave radars have to do primarily with larger antenna sizes required for antenna gains comparable to microwave; their sizes can be larger by as much as three orders of magnitude. Penalties of this antenna size are: (i) they become prohibitively costly or too impractical for most applications; or (ii) if antenna size is reduced, inadequate target detection sensitivity may result when standard radar signal formats are used. In addition, the narrower bandwidth of such radar signals makes them more susceptible to intercept and jamming.
The normal way a microwave radar determines target direction is to form a narrow beam. This is done with an aperture many wavelengths in extent. The beamwidth (in radians) is nearly the wavelength divided by the antenna length. When beam forming is used with HF skywave radars, for example, phased array antennas several kilometers in length are required. Narrow-beam surface-wave radars, such as the British OSCR for ocean current mapping [Prandle and Ryder, 1985] use phased array antennas that require more than 100 meters of lineal coastal access, a frequently impractical constraint. Antennas at HF with sizes the order of a wavelength in extent (e.g., 10-20 meters) have nearly omni-directional patterns, and are considered inadequate for accurate radar angle determination if beam forming and scanning are employed. An alternate way to determine angle is to employ direction-finding (DF) principles, which has not commonly been used with radars. U.S. Pat. Nos. 3,882,506 and 4,433,336 describe hardware implementations of two crossed single-turn air-loops and a monopole all mounted along the same axis. However, these loop antennas are still quite large, e.g., 1-4 meters across at mid-HF; it was believed that the antennas had to be highly efficient to provide adequate sensitivity and angle accuracy. The point being missed was that a receive antenna at lower frequencies does not have to be highly efficient, and therefore be large, in order to provide maximum possible radar sensitivity and accuracy. The reason is that external noise dominates, and the antenna need only possess an efficiency so that external and internal noise are comparable. Any size or cost increase to improve efficiency of the receive antennas beyond this point is wasted. The present invention goes beyond the hardware-only inventions of the above patents by giving algorithms for extracting angles, and allowing for more than three colocated orthogonal elements for radar signal DF.
Since both transmit and receive antenna gains of lower-frequency radars are less, target detection and location accuracy are worse if the same waveforms are used as for microwave radars. Microwave radars use pulse waveforms having low duty factors (the ratio of pulse width to pulse repetition interval), usually 1% or less. To gain back the sensitivity and accuracy lost by antenna size, lower-frequency radars have typically gone to high duty-factor signals. These radars are usually operated against moving targets, and Doppler processing is part of the waveform design and utilization. HF skywave radars, where the transmit and receive sites are separated by tens of kilometers, use 100% duty factor signals, i.e., transmitter and receiver are on all the time. Here the favored waveform has been the simple linearly swept frequency-modulated continuous-wave (FMCW) signal as described by Bartick [1973].
When the transmitter and receiver are colocated, as are in compact backscatter radars at HF, one cannot transmit and receive at the same time because the strong transmit signal overwhelms the receiver. Then the highest possible duty factor is 50%; i.e., the transmitter is turned off while receiving and vice versa. The design of efficient, non-ambiguity-producing waveforms and digital signal processing that combine high (e.g., 50%) duty factor pulsing/gating with modulation formats that give target range, such as linear FMCW, has not yet been successfully implemented. Three periodic processes are happening simultaneously: (i) the modulation used for target range determination, e.g., linear FMCW; (ii) the pulsing/gating process; (iii) the digital sampling occurring in the analog-m-digital (A/D) convertor. Each of these three periodic processes replicates the target echo in the frequency domain, and when all three happen at once, severe aliasing and ambiguities can result. For example, the simple linearly swept FMCW signal starts with a potential range-Doppler ambiguity. Attempting to mitigate this problem for high speed targets can cause Doppler aliasing, i.e., two or more possible choices for Doppler. To overcome this, one would increase the linear sweep repetition frequency, but then range aliasing can occur (two or more possible choices for target range). And the pulse/gate repetition frequency itself--if chosen improperly--can cause both range and Doppler aliasing. Attempts to eliminate the latter by shortening the pulse will either: (i) lower the duty factor, or (ii) result in blind zones, where targets will not be illuminated or seen. Although jittering of any of these repetition rates--as well as the frequency itself--can unravel the ambiguities and eliminate blind spots, these inelegant solutions add complexity and can make Doppler processing very difficult. Examples are U.S. Patent Nos. 4,896,159; 4,309,703; and 4,053,884.
Another disadvantage of conventional pulse-Doppler or chirp waveforms with time-domain pulse compression is the high digitizing and data-processing rates required. The A/D must generally sample at least twice the RF bandwidth, this latter bandwidth being dictated by the range resolution desired. Even for lower-frequency radars, the rates required are typically higher than 200 kHz per receiver channel. This precludes use of 16-bit convertors, with resultant limitations on signal dynamic range, and thence clutter and interference rejection. It also rules out use of widely available, commercial, inexpensive DSP (digital-signal-processing) boards.
A third problem with existing pulse radar design is the requirement for STC (sensitivity-time control) circuitry. These active circuits change the gain of the receiver circuits rapidly with time after transmission of each pulse, in order to flatten the dynamic range between very strong close-in clutter echoes and the most distant target echoes. The difference between these echoes can exceed 140 dB, far beyond the range of practical linear receiver operation. STC circuitry increases the cost and complexity of radars considerably, and hence incentive exists to eliminate this function.
It is often desirable to spread the radar signal's spectral energy over a very wide bandwidth. There will be less interference to others, and in military radars, it makes the signal less susceptable to detection/intercept and jamming. In the range-only (no Doppler) 100% duty-factor FMCW radar described in U.S. Pat. No. 5,023,618, this was a primary objective of their signal design and processing. Nevertheless, such FMCW signals, sweeping slowly and repetitively over a moderately wide bandwidth--as well as repetitive pulse signals--are both becoming easier to detect and jam with modem sophisticated military systems. Hence, a signal that can be used with a lower-frequency backscatter radar, provides high sensitivity, range, and Doppler information, avoids aliasing and ambiguities, has much lower digitizing rates than the RF signal bandwidth, and is difficult to intercept and jam, has not yet been implemented. The present invention reveals a waveform design methodology and its digital generation/processing that accomplishes these goals.